1. Field of the Invention
The present invention generally relates to shielding devices for minimizing electromagnetic interference and, more particularly, is concerned with an enclosure and an assembly employing the enclosure for providing effective shielding of circuit board compoents against electromagnetic interference.
2. Description of the Prior Art
Electronic components found in various types of transmitters, receivers, computers and other electronic devices emit or are susceptible to electromagnetic radiation. It is therefore desirable to shield circuit components to reduce undesirable electromagnetic interference (EMI) and/or susceptibility effects with the use of conductive shields that reflect or dissipate electromagnetic charges and fields. Such shields are generally grounded to allow electrical charges and fields to be dissipated without disrupting the operation of the electronic components enclosed within the shield.
Various types of shielding devices for electronic components mounted on printed circuit boards have been proposed in the prior patent art. Representative examples of such shielding devices are the ones disclosed in U.S. patents to Stickney et al (U.S. Pat. No. 4,754,101), Hibino et al (U.S. Pat. No. 4,841,414), McCoy, Jr. (U.S. Pat. No. 5,014,160), Rogers et al (U.S. Pat. No. 5,043,848), Johnson (U.S. Pat. No. 5,095,177), Fry et al (U.S. Pat. No. 5,160,807) and Moore (U.S. Pat. No. 5,175,395); a Japanese patent to Tsunoda (U.S. Pat. No. 62,595), and a German patent to Bardon (U.S. Pat. No. 3,736,833). These shielding devices frequently include a base frame which is mounted to the printed circuit board surrounding the circuit board components, and a cover which fits over and connects with the base frame enclosing the printed circuit board components.
Various difficulties may be encountered with respect to providing a satisfactory connection of the cover to the base frame. The connection must secure the cover to the base frame in a reliable manner which prevents dislodgement due to shock and vibration. At the same time, the connection must permit easy removal of the cover for periodic visual inspection of the printed circuit board and the components enclosed by the shield.
As an example, such difficulties may be experienced with respect to the electromagnetic shielding assembly of U.S. Pat. No. 4,754,101 to Stickney et al. The shielding assembly of this patent has a cover plate and a base frame with a plurality of walls made from a length or strip of metallic material. The base frame is formed in a rectangular configuration so as to surround the electronic component on the circuit board. The base frame has a plurality of spaced mounting pins projecting downwardly from a lower edge of the strip for alignment within holes in the circuit board. The base frame also has a plurality of spaced deflectable engagement prongs defined by spaced slots and extending upwardly from an upper edge of the strip. The prongs have flanged upper end to allow insertion of the cover plate within the base frame and outwardly formed recesses between the flanged upper ends to receive and hold the cover plate. The above-described design of the shielding assembly of the Stickney et al patent fails to provide any positive means for preventing inadvertent overinsertion of the cover plate downwardly past the recesses of the prongs which can result in overdeflection thereof and damage thereto and to the electronic component enclosed by the shielding assembly.
Consequently, a need still exists for improvement in the design of shielding devices for printed circuit board components.